The Subaru-Robin EX21D is a 211 cc (12.88 cu-in) single-cylinder air-cooled 4-stroke internal combustion small gasoline engine manufactured by Industrial Products division of Subaru Corporation for general-purpose applications.
The Subaru-Robin EX21D engine has an OHС (overhead camshaft) design; high compression ratio for better fuel efficiency; horizontal draft, float type carburetor; recoil starter or electric starter as option and transistorized magneto ignition system.
It has a 67.0 mm (2.64 in) cylinder bore and 60.0 mm (2.36 in) piston stroke. Compression ratio rating is 8.5:1. The engine produced 7.0 PS (5.1 kW; 6.9 HP) at 4,000 rpm of maximum horsepower and 4.4 PS (3.2 kW; 4.3 HP) at 3,000 rpm or 5.0 PS (3.7 kW; 5.0 HP) at 3,600 rpm of continuous output power. Maximum torque is 13.9 N·m (1.4 kg·m, 10.2 ft·lb) at 2,500 rpm.
General information
| Engine Specifications | |
| Model | EX21D |
| Type | 4-stroke, OHC, single cylinder |
| Displacement | 211 cm 3 (12.88 cu-in) |
| Max. horsepower | 7.0 PS (5.1 kW; 6.9 HP) at 4,000 rpm |
| Max. torque (crank PTO) | 13.9 N·m (1.4 kg·m, 10.2 ft·lb) at 2,500 rpm |
| Carburetor | Horizontal draft, float type carburetor |
| Cooling system | Forced-air |
| Ignition system | Transistorized magneto ignition |
| Lubricating system | Splash |
| Starting system | Recoil or electric starter |
| Stoping system | Ignition primary circuit ground |
| Fuel used | Unleaded gasoline (octane number 86 or higher) |
| Fuel tank capacity | 3.6 L (0.95 US. gal. 0.79 Imp. gal.) |
| Fuel consumption | 367 g/kWH |
| PTO shaft rotation | Counterclockwise (from PTO shaft side) |
Cylinder Block and Head
The EX21D has cast iron cylinder sleeve, ball bearing supported forged steel crankshaft, improved piston design. The piston has two compression rings and one oil ring.
| Cylinder block | ||
| Compression ratio: | 8.5:1 | |
| Bore x Stroke | 67.0 X 60.0 mm (2.64 x 2.36 in) | |
| Sleeve internal diameter | 67.000-67.019 mm (2.6378-2.6385 in) | |
| Piston skirt outer diameter | 66.960-67.000 mm (2.6362-2.6378 in) | |
| Number of piston rings (compression / oil): | 2 / 1 | |
| Ring side clearance | Top | 0.035-0.080 mm (0.0014-0.0031 in)) |
| Second | 0.035-0.080 mm (0.0014-0.0031 in) | |
| Oil | 0.010-0.065 mm (0.0004-0.0026 in) | |
| Ring end gap | Top | 0.15-0.30 mm (0.0059-0.0118 in) |
| Second | 0.20-0.40 mm (0.0079-0.0157 in) | |
| Oil | 0.05-0.25 mm (0.002-0.0098 in) | |
| Connecting rod small end ID | 16.010-16.021 mm (0.6303-0.6307 in) | |
| Connecting rod big end ID | 30.000-30.016 mm (1.1811-1.1817 in) | |
| Piston pin outer diameter | 15.992-16.000 mm (0.6296-0.6299 in) | |
| Crankshaft crankpin outer diameter | 29.967-29.980 mm (1.1798-1.1803 in) | |
| Crankshaft main journal diameter | №1 | 27.988-24.997 mm (1.1019-0.9841 in) |
| №2 | 24.988-24.997 mm (0.9838-0.9841 in) | |
| Cylinder Head | ||
| Warpage (limit) | 0.10 mm (0.004 in) | |
| Valve Arrangement: | OHC | |
| Valves: | 2 | |
| Stem outer diameter (standard): | INTAKE | 5.440-5.455 mm (0.2142-0.2148 in) |
| EXHAUST | 5.425-5.445 mm (0.2136-0.2144 in) | |
| Valve spring free length: | 27.4 mm (1.0787 in) | |
| Camshaft lobe height: | INTAKE | 29.028-29.128 mm (1.1428-1.1468 in) |
| EXHAUST | 29.028-29.128 mm (1.1428-1.1468 in) | |
Tightening torque specs
| Tightening torque specs | |
| Cylinder head M8 × 68 flange bolt (Re-use) | 25-27 Nm; 2.5-2.7 kg·m; 18.1-19.5 ft·lb |
| Cylinder head M8 × 68 flange bolt (New) | 28-30 Nm; 2.8-3.0 kg·m; 20.3-21.7 ft·lb |
| Cylinder head M8 × 35 flange bolt | 17-19 Nm; 1.7-1.9 kg·m; 12.3-13.7 ft·lb |
| Connecting rod bolt | 13-15 Nm; 1.3-1.5 kg·m; 9.4-10.8 ft·lb |
| Main bearing cover bolts | 22-24 Nm; 2.2-2.4 kg·m; 16.2-17.7 ft·lb |
| Flywheel nut | 59-64 Nm; 5.9-6.4 kg·m; 42.7-46.3 ft·lb |
Service Information
| Engine | |
| Cylinder compression | 3.5 kg/cm2 |
| Valve clearance | |
| Intake valve clearance | 0.12-0.15 mm (0.0047-0.0059 in) |
| Exhaust valve clearance | 0.12-0.15 mm (0.0047-0.0059 in) |
| Oil system | |
| Oil type | 4-stroke or an equivalent (SE or SF) |
| Recommended oil | SAE 10W-30 |
| Oil capacity | 0.6 L (0.63 US. qt, 0.53 Imp. qt.) |
| Ignition system | |
| Spark plug |
NGK: BR-6HS; CHAMPION: RL86C |
| Spark plug gap | 0.6-0.7 mm (0.0236-0.0276 in) |
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